The present invention relates to a microwave-excited discharge lamp which emits light by discharge under a microwave electromagnetic field.
Conventionally, in a liquid crystal projection display using a liquid crystal panel, an electrode discharge lamp including a metal halide lamp and a xenon lamp has been used as a light source. As is well known, in the light source of the liquid crystal projection display, a light output must be collimated through a lens for projection into the liquid crystal panel. Therefore, as the light source, it has been necessary to reduce the size of a light emitting part as much as possible in order to increase its light utilization. Furthermore, it has been required to retain the light output even when the size of the light emitting part is reduced. In existing electrode discharge lamps including the metal halide lamp, the size reduction of the light emitting part has been achieved by shortening a gap of electrodes thereof. However, in the case that the gap of the electrodes is shortened without reducing the light output, electric power applied to the electrodes inevitably becomes large in the electrode discharge lamp. As a result, the lifetime of the electrode discharge lamp has been extremely short (several thousand hours) compared with the lifetime required for a television monitor and the like. Various efforts have been made to date, but the electrode discharge lamp that can satisfy the brightness and lifetime requirements at the same time has not yet been developed or commercially implemented.
In recent years, an inherently long life electrodeless discharge lamp, which is free from electrode deterioration determining the above-mentioned lifetime of the electrode discharge lamp, has been attracting attention. One commercial implementation of the electrodeless discharge lamp is a microwave-excited discharge lamp which emits light by discharge under a microwave electromagnetic field formed by a microwave (in the 1 GHz to several tens of GHz band).
As a conventional microwave-excited discharge lamp, there is a description in IDW (International Display Workshop), 1996 version, pp. 435-438 ("Novel High Color Rendering Electrodeless HID Lamp Containing InX). This conventional microwave-excited discharge lamp uses a discharge tube with thickness about 1.5 mm and outer diameter 15, 20, 30, or 40 mm. Inside of the discharge tube is filled with argon (Ar) and an indium halide, namely, indium iodide (InI) or indium bromide (InBr).
When such conventional microwave-excited discharge lamp is used as the aforementioned light source instead of a short arc HID lamp, the discharge tube must be made smaller with its inner diameter reduced to about 3 mm to 8 mm. However, as is well known, in the microwave-excited discharge lamp, the smaller the discharge tube is made, the closer becomes the distance between the tube wall and the plasma discharge generated in the discharge tube, resulting in higher tube wall temperature. Accordingly, in the conventional microwave-excited discharge lamp, when the discharge tube is reduced in size, it has become necessary to cool the lamp in order to maintain stable operating condition, and it has also been necessary to control the lamp temperature with high accuracy.
It is known to seal mercury as a buffer gas within the discharge tube in order to maintain the stable operating condition. However, in the conventional microwave-excited discharge lamp, there is a problem of a low luminous efficacy when the amount of the buffer gas comprising mercury is increased. As a result, in the conventional microwave-excited discharge lamp, it is necessary that the amount of mercury to be sealed inside the discharge tube is extremely small. However, accurately sealing a very small amount of mercury into the discharge tube has been impracticable in mass production, though it may be possible in the laboratory. Therefore, it has been difficult to use the conventional microwave-excited discharge lamp as the aforementioned light source.